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555 Timer Astable / Monostable Calculator

Calculate frequency, period, duty cycle (astable mode), and pulse width (monostable mode) for the classic 555 timer IC based on R1, R2, and capacitor values.

In astable mode, R1 must be > 0. In monostable, only R1 is used.
Used only in astable mode. Increasing R2 decreases frequency and duty cycle.

Results

Frequency (Astable)480.00 Hz
Period (Astable)0.0,021 s
Duty Cycle (Astable)66.7%
Pulse Width (Monostable)0.0,011 s

📖What is it?

The 555 timer is one of the most popular ICs ever made. In astable mode it generates a continuous square wave: f = 1.44/((R1+2R2)�C), duty cycle = (R1+R2)/(R1+2R2). In monostable mode it produces a single timed pulse when triggered: t = 1.1�R1�C. Note: 50% duty cycle requires R1 << R2 or use of a bypass diode across R2.

🎯How to use

Select the mode, enter R1 and R2 in kO, and capacitance in �F. For monostable, only R1 and C matter. All four outputs (frequency, period, duty cycle, and monostable pulse width) are always calculated.

💡Example scenario

LED blinker at ~1 Hz, 50% duty: R1 = 1kO, R2 = 72kO, C = 10�F. f = 1.44/((1k+144k)�10�) = 1.44/1.45 � 0.99 Hz. Duty � (1+72)/(1+144) � 50.3%. For 1-second monostable pulse: R1 = 910kO, C = 1�F ? t = 1.1�910k�1� = 1.001s.

🏆Pro tip

The 555 cannot achieve exactly 50% duty cycle in standard astable mode because the charge path includes R1+R2 and discharge path is only R2. Use a Schottky diode in parallel with R2 (cathode toward pin 7) to bypass R1 during discharge, achieving ~50%. The CMOS version (TLC555) draws less current and is better for battery-powered applications.